This invention involves an improved method and apparatus for selectively shutting off the flow of molten plastic molding material to a mold, useful in applications such as injection molding.
In the well-known injection molding process, thermoplastic material is melted to form a viscous liquid which is then injected under pressure into a mold cavity where it cools and solidifies. Solidification is accompanied by volumetric shrinkage, therefore necessitating the maintenance of a high packing pressure during cooling in order to achieve dimensional accuracy of the molded parts. As the plastic in the mold cavity cools, the packing pressure can decline, without any resulting change in dimensions of the molded plastic part.
It is customary to provide some sort of gate or valve to shut off the plastic flow into the cavity once injection is completed and the packing pressure has been induced, in order to prevent plastic in the mold cavity, which is pressurized, from flowing back into the supply runner. Several techniques have been employed to accomplish this shut-off in the constricted or gate area immediately upstream of the mold cavity. One known technique is to cool the gate area so that the plastic in the gate area freezes once flow has essentially stopped due to filling of the cavity, thereby blocking further flow in either direction. This type of cooling system is continuous, providing an essentially constant temperature, so that no precise control of the plastic flow is possible.
Another general type of construction is the use of a movable pin placed in the flow path to close the gate. However, movable pins introduce substantial complexity accompanied by problems such as wear and misalignment of the pin and valve seat, thereby causing maintenance and reliability problems and expenses.
A thermal gate, which is presently being marketed by Spear System, Inc. of Chatsworth, Calif. and described in the U.S. Pat. No. 3,800,027, to Tsutsumi employs a stationary central axial pin in the constricted gate area. The pin has one heating element in its main body and a separately controlled heating element at its tip, the tip being located in the smallest portion of the flow passage. With the tip heater current on, to melt the gate open, plastic flows longitudinally along the length of the pin in the annular zone between the pin and the surrounding walls of the passage. When the tip heater current is turned off, the plastic freezes in the gate area. In the Tsutsumi construction, there are several disadvantages. Some of the flowing plastic passing through the gate area contacts a cooled passage wall, while other portions of the plastic contact the heated wire. The resulting lack of uniform thermal history is often highly undesirable. Secondly, the cooling rate of the Tsutsumi system will be limited by the fact that the wire can only cool by conveying its heat through the plastic to the outer cooled passage walls, because the remainder of the core pin stays hot.
Another patent, somewhat similar to Tsutsumi, is Yoshida, U.S. Pat. No. 4,516,927.
In conventional cooled gates, there is no precise control of the plastic temperature in the gate area. Inaccuracy in the control can result in premature freezing of the plastic in the gate before the mold is filled, commonly known as a "short shot". This problem can be remedied only by using higher pressures or higher temperatures. Higher pressures require the use of larger machines and result in higher residual stresses, whereas higher plastic and mold temperatures result in longer cycle times to cool the part, increasing production costs.
To avoid the waste of plastic in runner systems, which freeze along with the molded part and then have to be removed and recycled, it is common to continuously heat the supply manifold throughout the molding cycle. Plastic flow is constricted in the gate area, and only the very tip of the gate, immediately upstream of the cavity, is cooled. However, it is difficult to accurately confine the cooling area to the constricted gate, while maintaining the supply manifold adjacent thereto in a heated condition. Undesired continued heating in the gate area may cause the molten material in the molded part close to the gate to remain heated longer than the balance of the molded part. Because crystalline and semicrystalline plastic molding materials are very sensitive to their thermal history, this lack of precise temperature control in the gate area may result in undesirable physical properties of the resulting molded part.
While the heated probe type of thermal gate is an improvement over the prior art, the present invention has as its objective the further improvement of thermal gating systems by more accurate temperature control and more rapid heating and cooling in the gate area to further increase the efficiency and reliability of high speed production molding processes.
Laminations of materials possessing differing thermal conductivity properties have heretofore been proposed for use in injection molding molds, to improve the physical properties of the molded part. Exemplary of such prior art are the patents to Yotsutsuji et al, U.S. Pat. No. 4,225,109 (thin metal layer lining mold cavity, formed on layer of heat insulating material, to delay cooling of surface of molded part); Yang, U.S. Pat. No. 4,390,485 (thin metal layer of high electrical resistivity lining mold cavity to produce rapid heating thereof). Additionally, co-pending Unites States patent application Ser. No. 616,294 of Holden, Suh and Applicant Border discloses a variety of laminated constructions which are selected for their ability to heat rapidly by electric resistance heating and to cool rapidly upon termination of electrical current flow, with low thermal inertia and minimal thermal stresses. Such laminated constructions are suggested therein for use in controlling the thermal response of the surface of a mold cavity. However, none of these disclosures is concerned with or suggests the use of such constructions as part of a method or apparatus for starting or stopping the flow of molten molding material into the mold cavity, thus acting as a gate or valve.